Fast response capacitive gauging system featuring steep slope filter discrimination circuit

ABSTRACT

The high speed, high accuracy capacitive gauging system employs an oscillator fed through steep slope filter that is discriminates between very small changes in capacitance even in the presence of electrical noise. During intervals when the probe tip is retracted, the oscillator frequency is calibrated to match the sweet spot in the center of the linear operative region of the steep slope filter. This calibrates the system to overcome the effects of varying temperature and humidity in the manufacturing environment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/244,742, filed on Sep. 22, 2009. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates generally to capacitive gauging systemsand more particularly to an improved capacitive gauging system for usein applications such as industrial laser processing, including but notlimited to cutting, scribing, ablating and welding, and for use in otherapplications where the position of a moving member is controlled ormeasured using capacitive gauging. The system described herein canprovide a means of measuring the tip-to-part standoff distance, forexample. This distance measurement may be used as feedback to a servoloop to maintain a constant tip-to-part standoff, for example.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art. In capacitive gaugingsystems there are applications, such as laser processing, whereextremely fast response times, noise immunity and compensation ofbackground capacitance are essential. The ability of a system to excelin these features would make a product more effective and superior toother designs.

For example, maintaining a constant standoff height is advantageous in alaser process, as it keeps the laser focus consistent in relation to thepart, as well as keeping the gas flow dynamics consistent. Usingcapacitance to measure the tip-to-part standoff distance is thepreferred method for laser processing; however there exists somesignificant design challenges. Among these challenges are:

-   -   a) Fast laser processing results in proportionally fast        tip-to-part standoff compensation. Typical response times for        capacitive gauging systems are slower than 2 msec, and thus        place a practical limit on laser processing speeds.    -   b) Industrial laser processing is inherently electrically noisy.        The plasma and ejected molten metal can interfere with the        capacitance thus disrupting the ability of the system to        accurately measure the tip-to-part standoff. The typical way of        dealing with this interference is to filter it out with a low        pass filter with a cut-off frequency of ˜500 Hz., again        resulting in a relatively slow response time that degrades        overall laser processing speed.    -   c) The capacitance variation seen between the tip and the part        can be less than a picofarad. For this reason, keeping the        background capacitance constant is essential for accurate        tip-to-part gauging. However, variations in temperature,        humidity and other external influences can alter the background        capacitance, causing significant errors in the tip-to-part        measurement. Conventional systems have difficulty dealing with        this error.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

The improved capacitive gauging system described more fully herein has aunique analog circuit that employs a carefully calibratedcapacitance-responsive oscillator and a high-frequency, high-order, lowpass filter to process the output of the oscillator and thus measureminute changes in capacitance even when those changes occur veryrapidly. The improved gauging system has a potential response time offaster than 0.01 msec and is thus considerably faster to respond thanconventional (2 msec) capacitive gauging systems.

Instead of using a conventional 500 Hz. low pass filter to filter outlaser-induced noise (at the expense of speed), the improved capacitivegauging system uses its high-frequency, high-order, low pass filter torapidly discriminate between minute changes in capacitance. Thus theimproved capacitive gauging system concentrates on quick recovery fromnoise, instead of attempting to filter it out.

To address environmental variances (e.g., in temperature and/orhumidity) the improved capacitive gauging system uses a compensationcircuit to adjust for the background capacitance when the tip is arelatively far distance away from the part. The compensation circuit isengaged at intervals between the cutting, scribing, ablating and/orwelding processes. It accurately calibrates the oscillator to match the“sweet spot” of the high-frequency, high-order, low pass filter, so thatthe filter can continue to discriminate very small changes incapacitance as the probe tip-to-part distance changes.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a block diagram of the improved capacitive gauging systemshown in context with the servomotor system used to drive the up-downmovement of the tip relative to the part;

FIG. 2 is a detailed block diagram of the improved capacitive gaugingsystem, with superimposed graphs showing various relationships amongdistance (d), capacitance (C), frequency (freq), amplitude (amp) andvoltage out (Vout);

FIG. 3 is a graph showing how filter slope affects discriminationability of the capacitance sensor; and

FIG. 4 is an oscilloscope waveform diagram showing how the height (sensesignal) produced by the capacitive sensor compares with the positionindicator produced by the gauging system in the presence of noise.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

With reference to FIG. 1, the improved capacitive gauging system isshown generally at 10. As illustrated, the gauging system supplies acontrol signal to the motor driver circuit 12, which in turn controlsthe up-down motion of the tip servomotor 14. For illustration purposes,FIG. 1 shows the tip 16 in two positions: position 16 a—in workingproximity to the part 18 and position 16 b—in retracted position. Itwill be understood that during use, the servomotor 14 is controlled tomaintain the proper working distance (d) between tip 16 and part 18. Aswill be more fully described below, from time to time, such as betweenintervals of working interaction between tip and part (e.g., duringcutting, scribing, ablating and/or welding), the servomotor 14 retractsthe tip to position 16 b. When retracted to position 16 b, thecompensation calibration circuit 20 performs a calibration operationupon the capacitance sensor 22. Details of the compensation calibrationcircuit 20 and capacitance sensor 22 are provided below in connectionwith FIG. 2.

The improved capacitive gauging system employs a unique analog circuitthat performs highly accurate and rapid capacitive sensing. A digitalcircuit, cycle control processor 24 mediates the operation of thisanalog circuit. The cycle control processor 24 sends digital controlinstructions to the motor driver 12 to cause the tip servomotor 14 toretract the tip to position 16 b. This retraction motion would typicallybe performed after a particular working interaction between tip and parthave completed, such as when it is time to remove a finished part andinsert a fresh part to be worked upon. Upon retraction of the tip, thecycle control processor 24 sends digital control instructions to thecompensation calibration circuit 20, causing it to calibrate thecapacitance sensor 22 for its next use.

Referring now to FIG. 2, the capacitance sensor 22 of the improvedcapacitive gauging system 10 employs an oscillator 26. The frequencygenerated by the oscillator is a function of resistance (R), capacitance(C) and/or inductance (L). The capacitance (C) portion of the functionis itself a function of the distance (d) between the tip and the part.As the distance between the tip and the part changes, the capacitancechanges, resulting in a change in the oscillator frequency. Theresulting frequency is nominally between 1 KHz and 1 GHz. The oscillatorcan be based on a LM 555 timer circuit in “Astable operations” mode (seedata sheet www.natural.com/ds/LM555.pdf) optional modification to thiscircuit can be used to reduce the circuit's susceptibility totemperature variations. For example, the circuit can be separated intoan input buffer, comparator, and flip-flop.

The output of the oscillator 10 is sent through a high-frequency,high-order (steep slope) low-pass filter 28. The low pass filter 28 canbe implemented as active filter with a cut-off frequency close to theoperating frequency of the oscillator. In a presently preferredembodiment filter 28 is a 7^(th) order filter, which provides a verysteep cut-off curve. In general, the steeper the cut-off curve, the moresensitive the resulting system will be. The resulting output of filter28 will have an amplitude that increases as the frequency decreases.This can be illustrated with reference to FIG. 3, which shows the slopeof an exemplary low-pass filter. Note that the filter is designed withits “sweet spot” 30 centered at a frequency that matches the nominalfrequency of the oscillator 26 when the tip is in its retracted position16 b. Because of the steep cut-off slope, a small change in oscillatorfrequency (x-axis) will produce a large change in filter outputamplitude (y-axis). In order to achieve the high order (greater than 4)of filtering, commercially available active filters are implemented withappropriate support circuitry (e.g.,datasheets.maxim-ic.com/en/ds/max280-mxl1062.pdf).

While a high-frequency, high-order low-pass filter is presentlypreferred, the capacitance sensor 22 may also be implemented using ahigh-pass filter or notch filter. Essentially, any filter that providesa steep slope that can produce a large change in filter output for asmall change in oscillator frequency will work. If a high-pass filter ischosen, for example, the curve illustrated in FIG. 3 would be flipped sothat linear operative region would slope in the opposite direction.While this would change the polarity of the filter output, essentiallythe same frequency discrimination can be had.

To measure the output amplitude of filter 28, the capacitance sensor 22employs a radio-frequency detector 32. The radio frequency detector istuned to receive frequencies covering the frequencies associated withthe linear operative region of filter 28. The radio-frequency detector32 thus produces an output signal that, in effect, converts the varyingamplitude output of filter 28 into a varying voltage (V_(out)). The RFdetector 32 produces a DC voltage that is proportional to the power inthe signal that results from the bypass filter (low-pass) 28. The RFdetector can be implemented using a diode based rectifier circuitry ordedicated integrate circuit (seedatasheets.maxim-ic.com/en/ds/max2016.pdf). To improve signal response,a suitable gain stage 34 may be included, with its amplified output fedto a final output stage 36 which feeds and/or comprises part of themotor driver circuit 12 (FIG. 1). The motor output stage optimizes thesignal and conditions it for use as a gauging system. This signal thusfeeds the tip servomotor 14 which translates the V_(out) signal into aphysical position (d). The gain stage can be a standard op-amp baseddifferential filter.

To work effectively, the sweet spot 30 of the filter (FIG. 3) should bepositioned at or near the nominal frequency of the oscillator 26. Asexplained above, the oscillator frequency varies in relation to thesensed capacitance. Capacitance, in turn, varies as the distance betweentip and part changes. However, capacitance is also affected byenvironmental conditions, such as temperature and humidity, and theseenvironmental factors can significantly degrade accuracy. To compensatefor environmental variances, the improved capacitive gauging systememploys a compensation calibration circuit 20, which utilizes abackground capacitance compensator circuit 38. Compensator circuit 38,when engaged, forms a feedback loop between the output of gain stage 34and the oscillator 26. The compensator circuit 38 is selectivelyconnected (in response to instruction from the cycle control processor24, FIG. 1) when the tip is in its retracted position 16 b. Thebackground capacitance compensator circuit uses closed loop feedbackcontrol to accurately set the oscillator frequency of oscillator 26 byadjusting parameters of the oscillator frequency function to normalizethe resulting signal when the tip is far from the part, or some otherknown position.

Unlike conventional capacitive gauging systems, which employ heavy lowpass filtration (e.g. 500 Hz.) to filter out laser process-inducednoise, the improved capacitive gauging system is able to operate in thepresence of noise. To illustrate, refer to FIG. 4, which shows theoutput of a dual trace oscilloscope. The uppermost waveform depicts theoutput of the capacitive sense signal and the lowermost waveform depictsthe position indicator output of the gauging system. Note that thecapacitive sense signal has a lot of noise that is created by the laserprocess and is detected by the height sensing circuit. Even in thepresence of this noise, the resulting motion shown on the bottommostwaveform shows negligible noise.

From the foregoing it will be seen that the improved capacitive gaugingsystem advantageously utilizes a blend of analog and digital circuitryto produce fast and highly accurate measurements even in the presence ofnoise. The system uses the tip retraction intervals to performenvironmental calibration, thus ensuring high accuracy even in the faceof temperature and humidity variations.

While the improved capacitive gauging system has been illustrated anddescribed in its presently preferred form, it will be understood thatcertain modifications can be made to the circuitry or selectedcomponents without departing from the spirit of our inventive concept.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A capacitive sensing system for measuring theheight of a laser head above a work piece, said system comprising: alaser head nozzle forming a first plate of a capacitor; the work pieceforming a second plate of the capacitor; a capacitive-responsive radiofrequency oscillator operably coupled to the capacitor, said oscillatorproviding an output; and a high-frequency, high-order bypass filter toprocess the output from the oscillator to measure minute changes incapacitance of the capacitor.
 2. The capacitive sensing system of claim1, further comprising an actuator configured to move the laser headnozzle in response to the minute changes in capacitance of thecapacitor.
 3. The capacitive sensing system according to claim 1,wherein the high-order bypass filter has a response time of faster thanabout 0.01 msec.
 4. The capacitive sensing system according to claim 1,further comprising a temperature/humidity compensation circuitconfigured to compensate for changes in background capacitance.
 5. Thecapacitive sensing system according to claim 4, further comprising acontroller configured to engage the compensative circuit at intervalsbetween successive activation of the laser nozzle.
 6. The capacitivesensing system according to claim 5, wherein the successive activationof the nozzle is associated with one of cutting the work piece, scribingthe work piece, ablating the work piece, welding the work piece, andcombinations thereof.
 7. The capacitive sensing system according toclaim 1, wherein the bypass filter is one of a low-pass filter, ahigh-pass filter, and a notch-filter.
 8. The capacitive sensing systemaccording to claim 1, further comprising a radio-frequency detectorcoupled to the high frequency high-order bypass filter.
 9. Thecapacitive sensing system according to claim 8, wherein the radiofrequency detector converts a varying output of the filter into avarying voltage.
 10. A sensing system for a laser, the systemcomprising: a laser having a nozzle; an actuation mechanism coupled tothe nozzle; a capacitive sensing system comprising a capacitor formed ofthe nozzle and a work piece; a capacitive response oscillator operablycoupled to the capacitive sensing system, said capacitive responseoscillator varying from a first radio frequency to a second radiofrequency in response to changes in capacitance of the capacitivesensing system; a high-order bypass filter to process an output signalfrom the capacitive response oscillator, said high-order bypass filterbeing a greater than 4^(th) order bypass filter; and a radio frequencydetector configured to convert a varying output of the filter into avarying voltage, wherein said actuator mechanism is operably coupled tothe radio frequency detector to move the nozzle in response to thevarying voltage.
 11. The sensing system according to claim 10, furthercomprising a capacitive compensation circuit operably coupled to thebypass filter.
 12. The sensing system according to claim 11, wherein thecompensation circuit forms a feedback loop between an input of a gainstage and the oscillator.
 13. The sensing system according to claim 12,wherein the compensation circuit is selectively engaged when the nozzleis in an unengaged condition.
 14. The sensing system according to claim10, wherein the bypass filter is a high-order low-pass filter.
 15. Acapacitive gauging system for a laser, said system comprising: acapacitor formed from a structural portion of the laser and a workpiece; a radio frequency oscillator configured to detect changes incapacitance of the capacitor; a high-frequency high-order bypass filtercoupled to the oscillator, said bypass filter being a greater than4^(th) order filter to measure minute changes in an output of the radiofrequency oscillator; and a gain stage coupled to the radio frequencydetector, said gain stage configured to drive a laser head drivemechanism.
 16. The capacitive gauge system according to claim 15,further comprising a background capacitance compensator configured toadjust the output of the oscillator.
 17. The capacitive gauge accordingto claim 15, wherein the bypass filter is a low-pass filter.
 18. Thecapacitive gauge according to claim 15, wherein the drive mechanismcomprises a servo-motor.
 19. The capacitive gauge according to claim 18,further comprising a cycle control processor coupled to the drive motor.20. The capacitive gauge according to claim 15, wherein the radiofrequency oscillator comprises an input buffer, a comparator, and aflip-flop.